298 Lecture 15 
Since such a shape (but designed for a body of revolution) is designed on a 
potential-flow basis, the direction of flow can be reversed without influencing 
the pressure or velocity distribution. Thus it is perfectly feasible to design a 
body containing a fairly long region of low-velocity free-stream flow, a velocity 
gradient strongly favorable to the maintenance of extensive laminar flow, and a 
neck aft of this region which would screen at least some of the radiated noise 
from the body aft of this neck. Sucha shape has not been designed in detail since 
it is not known to be of any practical interest, but it could well be similar to that 
shown in Fig. 15.20, and it does indicate an application of aeronautics to under- 
water problems. It should be emphasized that suction is not necessary in this 
particular case since the discontinuity in velocity here is a favorable one, 
guarantees no separated flow and consequent high drag. In fact, some pressure 
measurements of such a wing, reversed and at incidence, were made many years 
ago by the principal author and are shown in Fig. 15.21. 
The extent to which sucha design is worthwhile depends on the strength of the 
radiated sound from the other parts of the structure, and this is difficult to 
calculate since we still do not know the basic source of such noise. 
We have implied that the radiated noiseis not likely to be that of the fluctuat- 
ing forces onthe body alone, but is more apt to be due to roughness or reradiation 
from panel oscillations. There is, therefore, little point at the moment in cal- 
culating the radiated noise from the pressure field in the way suggested earlier 
in the paper, but in any specific case such a calculation of possible magnitude 
would be enlightening and worth carrying out. 
15.5. CONCLUSION 
The foregoing paragraphs have shown in a qualitative manner that the radia - 
tion of the boundary layer under smooth wall conditions is by no means the 
largest contribution to the over-alllevel. Roughness and vibrating panel radiation 
are likely to be the largest components which can, in the widest sense, be esti- 
-mated at the present time. Skudrzyk's studies show that roughness will con- 
siderably enhance the high-frequency portions of the spectrum, while panel 
response might be expected to contribute largely to the low-frequency region. 
The authors appreciate that the estimates of cylinder vibration given herein 
ignore the more recently reported studies. It is hoped, however, that measure- 
ments of the actual response of submarine and torpedo structures on the lines 
indicated will be made available shortly, as they are obviously of considerable 
interest. 
Other aerodynamic sources, whose effects have not been taken into con- 
sideration, are fluctuating transition and separated and vortex flows. The latter 
two have received attention, as indicated earlier, from the point of view of 
aircraft structural design, but no measurements of radiation have yet been made. 
These details are shortly to be investigated at Southampton. 
Finally, it is hoped that the suggestion for a particular configuration of 
laminar flow noise might be of some merit in the reduction of noise received by 
acoustic homing sets while also providing a low-drag body acceptable also purely 
from the point of view of performance. 
